Graded and cross-reactive JAK-receptor interactions broaden the one-receptor, one-JAK rule

Cytokine receptors signal through a dedicated Janus kinase (JAK) that is thought to be constitutively bound to the receptor intracellular domain (ICD). We tested this one-receptor, one-JAK model biophysically. A bead-based screen of JAK1 and TYK2 against 32 receptor ICDs, confirmed by microscale thermophoresis to measure affinity, revealed dissociation constants spanning tens of nanomolar to tens of micromolar, indicating that many interactions are not high-affinity and may not be constitutive. Binding was also widely cross-reactive: GMCSFRα bound both JAK1 and TYK2, and IL2Rβ bound TYK2 far more tightly than JAK1. In cells, GM-CSF signaled through JAK1 when JAK2, JAK3, and TYK2 were inhibited. JAK-receptor pairing is therefore permissive and graded rather than exclusive and constitutive.